net_sched: cls_route: disallow handle of 0

net_sched: cls_route: disallow handle of 0

Follows up on:
https://lore.kernel.org/all/20220809170518.164662-1-cascardo@canonical.com/

handle of 0 implies from/to of universe realm which is not very
sensible.

Lets see what this patch will do:
$sudo tc qdisc add dev $DEV root handle 1:0 prio

//lets manufacture a way to insert handle of 0
$sudo tc filter add dev $DEV parent 1:0 protocol ip prio 100 \
route to 0 from 0 classid 1:10 action ok

//gets rejected...
Error: handle of 0 is not valid.
We have an error talking to the kernel, -1

//lets create a legit entry..
sudo tc filter add dev $DEV parent 1:0 protocol ip prio 100 route from 10 \
classid 1:10 action ok

//what did the kernel insert?
$sudo tc filter ls dev $DEV parent 1:0
filter protocol ip pref 100 route chain 0
filter protocol ip pref 100 route chain 0 fh 0x000a8000 flowid 1:10 from 10
	action order 1: gact action pass
	 random type none pass val 0
	 index 1 ref 1 bind 1

//Lets try to replace that legit entry with a handle of 0
$ sudo tc filter replace dev $DEV parent 1:0 protocol ip prio 100 \
handle 0x000a8000 route to 0 from 0 classid 1:10 action drop

Error: Replacing with handle of 0 is invalid.
We have an error talking to the kernel, -1

And last, lets run Cascardo's POC:
$ ./poc
0
0
-22
-22
-22

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>